powerpc/64s: Optimise MSR handling in exception handling

mtmsrd with L=1 only affects MSR_EE and MSR_RI bits, and we always
know what state those bits are, so the kernel MSR does not need to be
loaded when modifying them.

mtmsrd is often in the critical execution path, so avoiding dependency
on even L1 load is noticable. On a POWER8 this saves about 3 cycles
from the syscall path, and possibly a few from other exception returns
(not measured).

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 5afd03e5e8b865e3dc0b08b2293364296a884eff..51df82b610843d5c490741fa33d59afb5a01c08b 100644 (file)
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -139,7 +139,7 @@ END_FW_FTR_SECTION_IFSET(FW_FEATURE_SPLPAR)
 #ifdef CONFIG_PPC_BOOK3E
        wrteei  1
 #else
-       ld      r11,PACAKMSR(r13)
+       li      r11,MSR_RI
        ori     r11,r11,MSR_EE
        mtmsrd  r11,1
 #endif /* CONFIG_PPC_BOOK3E */
@@ -195,7 +195,6 @@ system_call:                        /* label this so stack traces look sane */
 #ifdef CONFIG_PPC_BOOK3E
        wrteei  0
 #else
-       ld      r10,PACAKMSR(r13)
        /*
         * For performance reasons we clear RI the same time that we
         * clear EE. We only need to clear RI just before we restore r13
@@ -203,8 +202,7 @@ system_call:                        /* label this so stack traces look sane */
         * We have to be careful to restore RI if we branch anywhere from
         * here (eg syscall_exit_work).
         */
-       li      r9,MSR_RI
-       andc    r11,r10,r9
+       li      r11,0
        mtmsrd  r11,1
 #endif /* CONFIG_PPC_BOOK3E */
 
@@ -221,13 +219,12 @@ system_call:                      /* label this so stack traces look sane */
 #endif
 2:     addi    r3,r1,STACK_FRAME_OVERHEAD
 #ifdef CONFIG_PPC_BOOK3S
+       li      r10,MSR_RI
        mtmsrd  r10,1           /* Restore RI */
 #endif
        bl      restore_math
 #ifdef CONFIG_PPC_BOOK3S
-       ld      r10,PACAKMSR(r13)
-       li      r9,MSR_RI
-       andc    r11,r10,r9 /* Re-clear RI */
+       li      r11,0
        mtmsrd  r11,1
 #endif
        ld      r8,_MSR(r1)
@@ -308,6 +305,7 @@ syscall_enosys:
        
 syscall_exit_work:
 #ifdef CONFIG_PPC_BOOK3S
+       li      r10,MSR_RI
        mtmsrd  r10,1           /* Restore RI */
 #endif
        /* If TIF_RESTOREALL is set, don't scribble on either r3 or ccr.
@@ -354,7 +352,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
 #ifdef CONFIG_PPC_BOOK3E
        wrteei  1
 #else
-       ld      r10,PACAKMSR(r13)
+       li      r10,MSR_RI
        ori     r10,r10,MSR_EE
        mtmsrd  r10,1
 #endif /* CONFIG_PPC_BOOK3E */
@@ -619,7 +617,7 @@ _GLOBAL(ret_from_except_lite)
 #ifdef CONFIG_PPC_BOOK3E
        wrteei  0
 #else
-       ld      r10,PACAKMSR(r13) /* Get kernel MSR without EE */
+       li      r10,MSR_RI
        mtmsrd  r10,1             /* Update machine state */
 #endif /* CONFIG_PPC_BOOK3E */
 
@@ -751,7 +749,7 @@ resume_kernel:
 #ifdef CONFIG_PPC_BOOK3E
        wrteei  0
 #else
-       ld      r10,PACAKMSR(r13) /* Get kernel MSR without EE */
+       li      r10,MSR_RI
        mtmsrd  r10,1             /* Update machine state */
 #endif /* CONFIG_PPC_BOOK3E */
 #endif /* CONFIG_PREEMPT */
@@ -841,8 +839,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
         * userspace and we take an exception after restoring r13,
         * we end up corrupting the userspace r13 value.
         */
-       ld      r4,PACAKMSR(r13) /* Get kernel MSR without EE */
-       andc    r4,r4,r0         /* r0 contains MSR_RI here */
+       li      r4,0
        mtmsrd  r4,1
 
 #ifdef CONFIG_PPC_TRANSACTIONAL_MEM